Electric system for electric furnaces



H. A. GREAVES AND H. ETCHELLS;

ELECTRIC SYSTEM FOR ELECTRIC FURNACES.

'APPLXCATION men SEPT. I2, 1918.

1,370,016. I PatentedMar. 1,1921.

To all whom it may concern UNITED STATES PATENT OFFICE.

HENRY ARNOLD GREAVES AND HARRY ETCI-IELLS, 0F SHEFFIELD, ENGLAND.

ELECTRIC SYSTEM FOR ELECTRIC FURNACES.

-. Be it known that we, v GREAVES and HARRY ETCHELLS, both subjects of the King of Great Britain,,residing, respectively, at 25 Raven road and 231 Oak brook road, Shefiield, in the county of York, England, have invented certain new and useful Improved Electric Systems for Elec-.

tric Furnaces, of which'the following is a specification.

Our invention relates to electric furnaces and consists in the practical application of three phase alternating currents to an electric furnace having three upper electrodes or a multiple of three upper electrodes and a conductive hearth which is an electrical resistor in one unit. In our British Patent No. 106,626, of March 1, 1916, we described the advantages which accrue from generating a proportion of the heat underneath the bath of liquid metal, by passing current throu h a hearth made of refractory material w ich acts as an electrical resistor and consequently as a heating unit. Such an arrangement is quite distinct from electric furnaces in which the hearth acts merely as a terminal for applying an electric current, and not as a resistor, and in which case conse uently no heat isgenerated.

ur invention will be better understood on reference to the accompanying drawings in which Figure 1 is a diagrammatic representation of an electric furnace constructed according to our invention.

Fig. 2 is a representation ofthe primary windings of the transformers.

Fig. 3 is a diagrammatic representation of the arcs maintained at the lower end of each of three top electrodes and the resistance in the furnace hearth.

Fig. 4 is a diagrammatic representation of an extended application of our invention.

Fig. 5 is a representation of the primary windings of the transformers shown in Fig. 4.

The same letters refer to similar parts throughout the several views.

According to our invention we provide our im roved form of furnace with three or a multiple of three electrodes, at, I), and c,

Specification of Letters Patent.

Application filed. September 12, 1918. Serial No. 253,706.

5 HENRY Armani) Patented Mar. 1, 1921.

each connected to one terminal of the secondary windings of three transformers ()A, ()B, and OC The other terminals of the secondary windings of the three transformers are connected to a common point 0.

The prlmar r windings of -."tlie trans-.

formers 0A, B, 00, are represented in Fig. 2 by be, ab, and cc, and are connected in delta.

The secondary windings OA, GB, DC, are arranged with an angle of 60 between 0A, and OB, and between OB, and O0, in what is sometimes called an inverted star.

Assuming the transformers to be of identical transforming ratios, and the electrodes to be in equal adjustment equal currents will pass through 0A, 0B, and (X), when the meshed primaries are excited by three phase currents, and the result is that the three phase system is balanced in the primary both in power and power factor.

The common resultant current of the three electrodes will pass through the bath of liquid metal and through the resistance hearth of the furnace to the common point 0. This resultant current will be equal to .twice the current passing through any one of the furnace hearth also tends to damp short circuit currents passing through the electrodes, and consequently steadies the load fluctuations of the furnace as a whole. A better power factor is thus more easily maintained, as it is not necessary to introduce so much reactance as in furnaces in which the resistive hearth is not a feature.

The advantages appertaining to our in- .vention will be better understood on reference to Fig. 3 of the drawings wherein the arcs maintained at the lower end of each electrode are diagrammatically represented by three equal resistances, a, y, and 2 and the resistance of the furnace hearth by a resistance w.

The currents supplied from the secondaigy windings of the three transformers OA, 0 and 00, arranged as previously described, pass through 0;, 3/ and z, and meet in a common conductor m, n, which may be taken to represent the bath of metal in the furnace. The currents then traverse the common resistance w, which is then in series with m, y, and z, individually between their respective sources of current origin A10, BO, and CO. The total resistance between the secondary terminals of each transformer is the same in each case, and as the secondary pressures are the same, the load must be the same on each transformer thus balancing the loads on each individual primary winding of the meshed transformers.

The resultant current through the common resistance w, is the vectorial sum of the currents through OA, OB, and OC, which is twice the current in any one of these windings.

In connection with the before described arrangement it is possible to extend the application to include a method of varying the amount of energy used for heating the furnace bottom. Such an arrangement is shown in Fig. 4 of the drawings. From this illustration it will be seen that in addition to the three transformers OA, OB, and 00, previously described, we introduce a fourth transformer OD, between the common terminal junction O, and the bottom electrode connection of the furnace.

The primary of this transformer OD, is put in series with the primary winding of OB. Referring to Fig. 5, if a, (Z represents the primary of OB, then (Z, 6, represents the primary of OD. By varying the point d, the secondary voltage OD, can be varied. The secondary winding OD, is constructed to carry twice the current which flows through any one of the electrodes connected to A B and C, and thus at any instant the resultant current through OB, OC and OA will pass through DO.

The combined kilowatt absorbed by OD and OB should equal that absorbed by OA or 00 in order to keep the primary meshed windings balanced. This is accomplished by cutting down the secondary voltage OB until kilowatt OB-l-kilowatt OD kilowatt OA.

Thus if O current passing through any one electrode and electrodes connected to A and C are working at volts with the bath,

and 10 volts is desired across the hearth of 60 the furnace 60 C=10 2C+G (volts on B).

Volts on 13:60-20 :40 volts.

It will be seen that if this electrode B is put in the middle of the furnace, its lower voltage will compensate for less radiation from its arc, in proportion to the two outer electrodes.

It should be understood that the term transformer is equally applied to separate single phase windings of a compound transformer, which in effect is only a number of separate transformers compounded into one.

We wish it to be understood that we'do not claim the known method of inverted star connection of transformers and also we are aware that it has been previously known to construct hearths in separate resistive units, but

What we claim and desire to secure by Letters Patent of the United States is 1. An electrical furnace having three or a multiple of three upper electrodes and a hearth forming an electrical resistor in one unit combined as described with the system of three phase transformer grouping known as the inverted star and with means for varying the amount of electrical energy used for heating the furnace hearth consisting in the addition of a fourth transformer between the common terminal junction of the secondary windings of the original three transformers and the hearth electrode connection of the furnace in the manner and for the purpose substantially as herein described.

2. An electrical furnace having three or 100 a multiple of three upper electrodes and a hearth forming an electrical resistor in one unit combined as described with the system of three phase transformer grouping known as the inverted star and with means for 105 varying the amount of electrical energy used for heating the furnace hearth consisting in the addition of a fourth transformer between the common terminal junction of the secondary windings of the original three 110 transformers and the hearth electrode connect-ion of the furnace, such transformer to be excited by the same phases of electrical primary supply which excite the middle transformer secondary winding of the in- 1 5 verted star group.

LB NRY ARNOLD GREAVES. HARRY ETOHELLS. 

